High strength alloy for use at elevated temperatures



tates Patented July 8, 1958 HIGH STRENGTH ALLQY FGR USE AT ELEVATED TEMPERATURES Dolph G. Ebeling, Schenectady, N. Y, assignor to General Electric Company, a corporation of New York No Drawing. Application October i, 1954; Serial No. 459,832

6 Claims. (Cl. 75-171) The present invention relates to alloys which have high strength at temperatures up to and exceeding 1600 F, and wrought articles made from such an alloy.

The development of the gas turbine and similar apparatus has created a demand for materials having higher strength and greater stability at elevated temperatures than the high temperature alloys currently available. Certain components of such apparatus must be capable of withstanding very high stresses over long periods of time in highly corrosive atmospheres at continuous temperatures of the order of 1500 F., or more. the presently available alloys having high strength at elevated temperatures are casting alloys which can be forged, rolled or machined only with great difficulty. Since the apparatus components made from these alloys require close manufacturing tolerances, an alloy which can only be cast poses many difiiculties, only one of which is the difiiculty of maintaining a high degree of uniformity of quality in known precision casting procedures. A wrought alloy having the necessary high temperature strength would not only solve many manufacturing problems but also provide a better and less expensive product. The term wrought alloy is use to describe alloys which may be fabricated into finished articles by forging or rolling, to distinguish from alloys which, because of their inherent resistance to deformation without fracture, are only suitable for casting.

Many of the previously known wrought alloys have exhibited high initial strengthat elevated temperatures but, after a relatively short period of time under these conditions, have lost certain desirable properties, particularly strength and ductility. This loss of strength and ductility as a function of time and temperature has been attributed in part to an inherent physical instability of these alloys. It has been observed that these undesirable changes in mechanical properties are nearly always accompanied by significant changes in the microstructure of the alloys. These changes are usually a significant in crease of grain size or the precipitation of intermetallic compounds and phases, particularly at the grain boundaries, or, frequently, both. Therefore, it appears desirable that an alloy which is to retain high strength over long periods of time at high temperatures be resistant to grain growth and intergranular precipitation at elevated temperatures.

A further problem encountered in the operation of such apparatus is chemical instability of materials exposed to corrosive atmospheres at the operating temperature and stress. Inasmuch as one of the applications contemplated for my alloy is gas turbine buckets and blades, such an alloy should be resistant to a severely oxidizing atmosphere at elevated temperatures and should not excessively spall or scale.

The principal object of this invention is the provision of an alloy which has high strength at elevated temperatures, which is both chemically and physically stable under high temperature and stress, and which may be Many of readily fabricated by the usual methods of forging, rolling and machining.

A further object of this invention is the provision of wrought articles which at 1600 F. will withstand stresses of substantially the same magnitude as similar articles made of known alloys withstand at 1500 F.

A further object of this invention is the provision of an alloy possessing high strength and stability at ele vated temperatures and comprising one of a group of different, but similar elements within a particular effective concentration range, or equally advantageously including any number of these elements in any proportion, provided the total eifective concentration of the combined elements remains within the particular effective concentration range.

Many attempts have been made to produce a wrought alloy having good mechanical properties at 1500 F. Two accepted standards of comparison for judging such alloys are their stress rupture properties and fatigue strength. Rupture strength is usually expressed as the constant load in pounds per square inch (p. s. i.) necessary to break a test specimen of the alloy under consideration in tension in a given length of time at a given constant temperature. The fatigue strength of an alloy as herein used is expressed as the limiting stress in p. s. i. which when applied cyclically will cause failurev in million cycles at a given constant temperature.

One of the better commercial wrought alloys currently available is composed of approximately 44 percent, by weight, cobalt, 20 percent nickel, 20 percent chromium, 4 percent tungsten, 4 percent. molybdenum, 4 percent niobium with the balance iron. The fatigue strength of this commercial alloy for 18 cycles tested at 1200 F. and 1500 F. is 60,500 p. s. i. and 35,500 p. s. i., respectively. The rupture strength of this alloy for 100 hours at 1500 F. averages about 23,600 pounds per square inch, for 1000 hours at 1500 F. about 18,000 pounds per square inch or less and for 1000 hours at 1600 F, about 10,500 pounds per square inch, minor variations occurring between different heats or batches depending upon the amount and character of the impurities the heat treatment, and upon how closely the actual composition compares with that cited above. The fatigue and stress rupture strengths given for this alloy represent test results obtained from test s ecimens which had been forged or otherwise wrought and ground from ingots and therefore are representative of the wrought properties of this alloy. A comparison of the rupture strengths of this material at the two temperatures reveals a very significant and undesirable loss of strength between 1500 and 1600 F. The importance of this loss of strength in this temperature range may be illustrated by stating that if the operating temperature of the turbine buckets in any of the aircraft turbojet engines now in use could be raised 100 F. Without any attendant loss of strength, the thrust of these engines would be increased about i0 p eat.

The alloy of my invention is composed of cobalt, nickel, chromium, iron and carbon, with hardening and strengthening additions of titanium and one or more of the elements tungsten, molybdenum, niobium and tantalum.

The titanium content of this alloy is most effective as a strengthening agent when precipitated as a finely dispersed intermetallic compound by appropriate heat treatment. For example, in the alloy of my invention, the titanium is first put into solid solution by heating the alloy to an appropriately high temperature and then aging at a lower temperature. During the aging anneal, an intermetallic compound such as NlgTl, for example, is precipitated. If the effective titanium content of such an alloy is reduced due to substantial portions of the titanium being tied up as titanium oxide, nitride, carbide or like compounds, present as inclusions, less of the titanium can form the hardening and strengthening constituent upon heat treatment and therefore a. weaker alloy will result.

At the high temperatures necessary to melt the various constituents of alloys of this type, titanium has an exceedingly high rate of reaction with certain of the gases of the atmosphere, rendering it extremely difficult to alloy more than about 2.5 percent, by weight, of titanium in an air melt. Therefore, in order to prevent the dissipation of any substantial part of the titanium during melting by reaction with the atmosphere, it is preferred that a vacuum melting process equivalent to that described in U. S. Patent 2,564,498 grantedto I. D.

Nisbet (assigned to the assignee of the present application) be employed. This patent discloses a melting schedule in which the metal is melted in a vacuum furnace, subjected to a first deoxidation treatment using hydrogen, carbon or the like, and a final deoxidation treatment by the addition of titanium or zirconium and is then cast, all the recited steps beingperformed under a continuous vacuum.

I have discovered that the elements of the group consisting of tungsten, molybdenum, niobium and tantalum have nearly equivalent strengthening effects upon the basic composition of my alloy. In fact, within the limits to be later specified, any of these four elements constitutes substantially the full strengthening equivalent of any of the others. This equivalency extends to mixtures or combinations of two, three or four of the elements, provided that the aggregate proportion of these elements to the other constituents of the alloy is maintained the same on an atom-to-atom basis. Since the limitation-on the content of the strengthening agent titanium imposed by air melting is non-existent under the cited vacuum melting process, my alloy may contain more than 3 percent, by weight, titanium.

In order to correlate and evaluate the elfects of the various additions to the basic alloy, the compositions of the alloys of my invention are set forth in atomic percent since the elements comprising the variants have, of course, different atomic weights and react with the other elements of the alloys on an atom-to-atom basis. For example, one atom of molybdenum has a relative weight of 95.95, while one atom of tungsten has a relative weight of 183.92, and yet with reference to the other materials of the alloy, each of the atoms has otherwise substantially the same strengthening effect of the other. It will, therefore, be understood that hereafter in the specification and claims, percent means atomic percent and not weight percent. Inasmuch as the relationship and interconversion between atomic and weight percent is merely an arithmetic operation well known to those skilled in the metallurgy'art, no further discussion is deemed necessary.

My invention comprises an alloy having a composition within the range of up to percent iron, 20 to 40 percent nickel, 15 to 45 percent cobalt, 15 to percent chromium, 0.01 to 2 percent carbon, 3 to 9 percent titanium, and 0.5 to 7 percent of any one of the group tungsten, molybdenum, niobium and tantalum or any combination of the elements in this group, the aggregate amount of such a combination falling within the limits of 0.5 to 7 percent, and preferably an alloy having a composition within the range of 25 to percent nickel, 25 to percent cobalt, 20 to 25 percent chromium, 0.5 to 1.5 percent carbon, 4 to 7 percent titanium, a maximum of 20 percent iron and 1 to 3.5 percent of any one of the group tungsten, molybdenum, niobium and tantalum or any combination of the elements in this group. A particularly strong alloy in this range is one composed of about 2 percent iron, 28.5 percent nickel, 37.5 percent cobalt, 23 percent chromium, 1 percent carbon, 5.5 percent titanium, the balance comprising one or more of the elements tungsten, molybdenum, niobium H fiuenced by heat treatment.

and tantulum, but preferably tungsten, as will be more particularly disclosed later.

This alloy is of austenitic constituency and has excellent corrosion resistance at elevated temperatures.

In order to better illustrate and disclose the many advantages which my invention offers the art of high temperature, high strength alloys and wrought articles for use under high temperature and high stress, various tests were performed. A large number of heats or batches of my alloy were made and so-tested.

Specifically, test specimens of my alloy were prepared by hot forging bars from cast ingots, swaging and grinding the bars into conventional test specimen configurations and tested according to accepted practices for smooth and notched bar stress-rupture properties, smooth and notched bar fatigue strength, tensile strength, proportional limit, ductility and hardness. The notched bar tests differ from the smooth bar tests previously described only in that the test specimen is provided with a notch in its fracture zone. The results of this test are compared to the results of the smooth bar tests to evaluate the test material's resistance to the weakening effect of surface cracks and imperfections. The tensile strength and proportional limit are expressed in pounds per square inch and represent, respectively, the short time stress required to break a test specimen in tension and the short time stress necessary to produce plastic deformation in a tensile test specimen. Ductility is expressed in percent elongation of a test specimen which has been broken in tension. Hardness is expressed in arbitrary numbers obtained from a well-known commer-' cial testing apparatus, the hardness numbers increasing with increasingly hard materials. I have discovered the hardness and strength of my alloy are not greatly in- To illustrate the effect of heat treatment on the above-mentioned mechanical properties, the following typical test results are tabulated, from which it is readily apparent that while the alloy exhibits some age-hardening characteristics, it is quite stable at temperatures of the order of 1500" F. and above.

TABLE I Rockwell hardness, A scale In order to illustrate the effect of varying the chemical constituency of my alloy upon its mechanical properties, the following examples may be compared.

TABLE II Chemical analyses [Atomic percent] Ni Cr \V hi0 Nb 'li C 32. 4 22. 3 c 0. 46 0. 1 32. A 22. 5 5. 01 0. 55 32. 4 22. 5 1. b4 4. 76 0.51 32. 2 22. 7 0. 4. 84 0. 51] 28. 4 22. 7 4. S5 0. 53 28. 4 22. 8 26 5. l. 10 29. 0 22. 8 2. 37 5. 84 0. 85 28. 4 22. 3 3. 9d 4. 36 0. 51 28. 5 22. 5 2. 05 5. 01 0. il 28. 9 22. 8 l. 33 2. 99 0. 15 29. 4 23. 4 2. 40 4. 12 0. 84 31. 1 22. 4 1. 49 4. 35 0. 87 30. 9 22. 8 2. 42 4. 19 0. S8

TABLE H Mechanical properties 1 See Table VI.

It will be observed from the foregoing Tables II and III that heat A is very similar in composition to the other listed heats, differing principally therefrom in that it does not contain tungsten, molybdenum, niobium or tantalum. It will be noted also, that the 1500 F. 100 hour stress-rupture strength of heat A is almost identical with that of the reference alloy, i. e. 24,000 p. s. i. compared to 23,600 p, s. i. The strengthening equivalency of the additions of tungsten, molybdenum and niobium on an atom-for-atom basis is clearly apparent from these testing results. For example, theequivalency of molybdenum, tungsten and molybdenum plus tungsten additions are shown, particularly with reference to heats A, B, C and D. Similarly, the equivalency of molybdenum, tungsten and niobium is illustrated by a comparison of heats E, F, G, H, I and J.

The data in Table III is further illustrative of the improvement in properties, particularly stress-rupture strength of the alloy of my invention over an alloy which has not been strengthened with tungsten, molybdenum, niobium or tantalum or mixtures thereof.

In. order'to more particularly illustrate and disclose the mechanical properties of the alloy of my invention, test results of heats F and G are given in greater detail in the following tables.

TABLE IV Stress Rupture data [Smooth bar (solution treated 2,150 F., aged 1,350 F.)]

[Smooth bar (solution treated 2,150 11., aged 1,650 F.)]

20,000 1s 26 1,700 15, 000 64 24 10,000 300 16 1, 600 25, 000 57 22. 4 F 8 1 i 5,0 0 00 1 30, 000 281 17 25,000 1,105 19 350 138 74 9 e i 3g, g 1 1, 25%

[Notched bar (solution treated 2,150 F., aged 1,650 F.)]

1 Test terminated, sample not yet broken.

TABLE V Fatigue tests [Smooth bar (solution treated 2,150 F., aged 1,350 F.)]

Temp., Stress, Cycles to Heat F. p. s. i. Failure,

[Smooth bar (solution treated 2,150 F., aged (5) 1,500 F.)]

55, 000 2. 83 1,500 50,000 11.08 F 47, 500 13. 61 1,200 75, gOO 0. 24 55, 00 6.07 G 11500 50,000 50. 10

[Notched bar (solution treated 2,150 R, aged 1,500 F.)]

[Smooth bar (solution treated 2,150 E, aged 1,650 I1] [Notched bar (solution treated 2,150 F., aged 1,650 F.)]

1 These stresses are average stresses at the notched section and should be multiplied by a factor of 2.6 to obtain maximum stress at the base of the notch.

2 Tests terminated, sample not yet broken.

TABLE VI Tensile test data [(Solution treated 2,150 11, aged 1,350 F.)]

1 Approximate values.

From these and other testing results, it was determined. that a temperature of 2150 F. is adequate for solution treatment, and that the aging temperature should be 1650 F. for best ductility with little sacrifice in strength. This is best shown by comparing the values shown in Table IV, which shows stress rupture data and Table V, which shows fatigue test results for samples aged at 1350 F., 1500 F., and 1650 F. Table V1 gives tensile properties for samples aged at 1350 F. and 1650 F. These results show the improvement in ductility with a comparatively small corresponding decrease in other properties resulting from the increased aging temperature of 1650 F.

From the test data, the following assessment has been made of several characteristics of the alloy:

Stress rupture.The data obtained for the specimens solution treated at 2150 F. and aged at 1350 F. disclosed a 1000-hour, 1500 F. strength of about 25,000 p. s. i. Specimens solution treated at 2150 F. and aged at 1650 F. to produce higher ductility had a 100-hour, 1500 F. strength of about 35,000 p. s. i. and a 1000- hour, 1500 F. strength of about 25,000 p. s. i.

Fatigue strength.-Smooth bar specimens solution treated at 2150 F. and aged at 1350" F. and tested at 1500 F. had an extrapolated strength at cycles of about 45,000 p. s. i.

Smooth bar specimens solution treated at 2150 F. and aged at 150 F. and tested at 1500 F. had an extrapolated strength at 10 cycles of about 45,000 p. s. i. Smooth bar specimens solution treated at 2150 F. and aged at 1650 F. and tested at 1500 F. and 1200 F. had strengths at 10 cycles of about 45,000 p. s. i. and 55,000 p. s. i., respectively. Notched bar specimens solution treated at 2150 F. and aged at 1500 F. and 1650 F. had strengths at 10 cycles of about 20,000 p. s. i. and well above 20,000 p. s. i., respectively when tested at 1500 P.

Tensile strength.Specimens solution treated at 2150 F. and aged at 1350 F. had an ultimate tensile strength of about 90,000 p. s. i. at 1500 F. with about 1.5 percent elongation. identically treated specimens tested at 1600 F. had an ultimate tensile strength of about 75,000 p. s. i. with about 7.0 percent elongation. Subsequent investigation indicated that aging at 1650 F. after a solution treatment of 2150 F. gave an ultimate tensile strength at 1500 F. of about 85,000 p. s. i. with about 8 percent elongation.

in addition to the numerous heats produced by the vacuum melting technique, several similar heats were also air-melted. Test specimens were prepared from the airmelted heats and tested. The results of these tests showed a decline of all pertinent properties. For example, the 1000 hour 1500 F. rupture strength was only about 16,000 p. s. i.

When a comparison of the chemical analyses of the many heats prepared and tested was made with their respective mechanical properties, a correlation between the carbon content and the properties was noted. In general, heats which contained less than about 1 atomic percent carbon tended to have somewhat lower mechanical properties than those heats which contained about 1 or more atomic percent carbon.

As was previously noted, the commercially available high temperature wrought alloy described previously, suffers a severe loss in strength when the operating or testing temperature is increased from 1500" F. to 1600 F. This may be shown by a comparison of its stress rupture strengths for 1000 hours at the two temperatures. At 1500 F., its rupture strength is 18,000 p. s. i. and at 1600 F., about 10,500 p. s. i. The alloy of this invention has a 1000-hour rupture strength at 1500 F. of about 25,000 p. s. i. and at 1600 F, about 15,000 p. s. i. From this it may be seen that my alloy is about 7000 p. s. i. stronger at the lower, and about 5000 p. s. i. stronger at the higher temperature than the above-mentioned commercially available alloy.

A similar comparison of the fatigue strengths at 10 cycles at 1200 F. and 1500 F. of the commercial alloy and the alloy of this invention reveals substantially equal strengths at 1200 F., but that at 1500 F, the fatigue strength of the alloy of this invention exceeds the fatigue strength of the commercial alloy by about 10,000 p. s. i.

Thus, it is readily apparent that a wrought alloy has been provided which is significantly stronger at the temperature range 1500 F. to 1600 F. than has been known before.

A further advantage inherent in this alloy is the flexibility of its composition. inasmuch as some or all of the elements of the group consisting of tungsten, molybdenum, niobium and tantalum may be classified as strategic materials, and, therefore, their availability and cost are subject to controls other than the usual laws of economics, it is of importance that this alloy may be made with any one of these elements, or any combination of these elements, provided the substitutions and combinations are made atom-for-atom and not weight-forweight. This, of course, permits the manufacturer to select those ingredients available at the lowest cost of production.

It should be further noted, particularly with regard to the titanium content, that the preferred vacuum melting technique provides an alloy having very low amounts of inclusions such as metallic oxides and nitrides. These impurities exist in weight percentages of the order of 0.002 percent oxygen and 0.0005 percent nitrogen. This is important because of the extremely debilitating effect of appreciable amounts of titanium oxides and nitrides upon alloys containing over 2.5 weight percent titanium.

It is to be understood that small amounts of elements other than those specifically disclosed will inevitably be present in this or any other commercially produced alloy. However, these elements are present only as impurities and as such, do not materially alter the composition of the alloy as claimed. a

While I have discussed my alloy as having particular utility in the manufacture of turbine buckets or blades for aircraft type turbojet engines, such description is merely illustrative and by way of example, and I do not desire or intend that the alloy be limited to such a use.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A metal alloy having high mechanical and physicochemical properties at elevated temperatures consisting essentially of 28 to 32.5 atomic percent nickel; 36 to 38.5 atomic percent cobalt; 22 to 23 atomic percent chromium; 4 to 6 atomic percent titanium; 0.5 to 1.5 atomic percent carbon; from 1.8 to 4.5 atomic percent of a metal selected from a group consisting of tungsten, molybdenum, niobium and tantalum and mixtures of said metals; and the remainder of said alloy being substantially all iron with the proportions of the several constituents being so selected that the iron content does not exceed 2.5 atomic percent.

2. A metal alloy having high mechanical and physicochemical properties at elevated temperatures consisting essentially of 28 to 32.5 atomic percent nickel; 36 to 38.5 atomic percent cobalt; 22 to 23 atomic percent chromium; 4 to 6 atomic percent titanium; 0.5 to 1.5 atomic percent carbon; 1.8 to 4.5 atomic percent molybdenum; and the remainder of said alloy being substantially all iron with the proportions of the several constituents being so selected that the iron content does not exceed 2.5 atomic percent.

3. A metal alloy having high mechanical and physicochemical properties at elevated temperatures consisting essentially of 28 to 30 atomic percent nickel; 36 to 38.5 atomic percent cobalt; 22 to 23 atomic percent chromium; 4 to 6 atomic percent titanium; 0.5 to 1.5 atomic percent carbon; 1.8 to 4.5 atomic percent tungsten; and the remainder of said alloy being substantially all iron with the proportions of the several constituents being so selected that the iron content does not exceed 2.5 atomic percent.

4. A wrought article having a stress-rupture life of 100 hours or more under a constant load of 31,000 pounds per square inch at a temperature of 1500 F., said article being composed of an alloy consisting essentially of 28 to 32.5 atomic percent nickel; 36 to 38.5 atomic percent cobalt; 22 to 23 atomic percent chromium; 4 to 6 atomic percent titanium; 0.5 to 1.5 atomic percent carbon; from 1.8 to 4.5 atomic percent of a metal selected I from the group consisting of tungsten, molybdenum,

niobium and tantalum and mixtures of said metals; and

the remainder of said alloy being substantially all iron with the proportions of the several constituents being so selected that the iron content does not exceed 2.5 atomic percent.

5. A wrought article having a stress-rupture life of 100 hours or more under a constant load of 32,500 p. s. i. at a temperature of 1500 F., said article consisting essentially of 28 to 32.5 atomic percent nickel; 36 to 38.5 atomic percent cobalt; 22 to 23 atomic percent chromium; 4, to 6 atomic percent titanium; 0.5 to 1.5 atomic percent carbon; 1.8 to 4.5 atomic percent molybdenum; and the remainder of said alloy being substantially all iron with the proportions of the several constituents being so selected that the iron content does not exceed 2.5 atomic percent.

'6. A wrought article having a stress-rupture life of 100 hours or more under a constant load of 3 3,500 p. s. i. at a temperature of 1500 F., said article being composed of an alloy consisting essentially of 28 to 30 atomic percent nickel; 36 to 38.5 atomic'percent cobalt; 22 to 23 atomic percent chromium; 4 to 6 atomic percent titanium; 0.5 to 1.5 atomic percent carbon; 1.8 to 4.5 atomic percent tungsten; and the remainder of said alloy being substantially all iron with the proportions of the several constituents being so selected that the iron content does not exceed 2.5 atomic percent.

References Cited in the file of this patent FOREIGN PATENTS 510,154 Great Britain July 24, 1939 721,888 Germany June 23, 1942 263,074- Switzerland Nov. 1, 1949 492,837 Canada May 12, 1953 

1. A METHOD ALLOY HAVING HIGH MECHANICAL AND PHYSICOCHEMICAL PROPERTIES AT ELEVATED TEMPERATURES CONSISTING ESSENTIALLY OF 28 TO 32.5 ATOMIC PERCENT NICKEL; 36 TO 38.5 ATOMIC PERCENT COBALT; 22 TO 23 ATOMIC PERCENT CHROMIUM; 4 TO 6 ATOMIC PERCENT TITANIUM; 0.5 TO 1.5 ATOMIC PERCENT CARBON; FROM 1.8 TO 4.5 ATOMIC PERCENT OF A METAL SELECTED FROM A GROUP CONSISTING OF TUNGSTEN, MOLYBDENUM, NIOBIUM AND TANTALUM AND MIXTURES OF SAID METALS; AND THE REMAINDER OF SAID ALLOY BEING SUBSTANTIALLY ALL IRON WITH THE PROPORTIONS OF THE SEVERAL CONSTITUENTS BEING SO SELECTED THAT THE IRON CONTENT DOES NOT EXCEED 2.5 ATOMIC PERCENT. 